Permissive hypercapnia: what to remember

A prospective cohort study comparing monitored anesthesia care and intubated general anesthesia in cardiac surgery involving cardiopulmonary bypass

BACKGROUND

The aim of this study is to assess the feasibility and safety of monitored anesthesia care (MAC) versus intubated general anesthesia (IGA) for patients undergoing elective cardiac surgery with cardiopulmonary bypass (CPB).

METHODS

This prospective observational study included patients scheduled for cardiac surgery involving CPB at our institution between April 2012 and February 2017. The enrolled patients were categorized into MAC and IGA groups. MAC involved local anesthesia at the sternotomy site, sedation with dexmedetomidine, analgesia with remifentanil/sufentanil, and electroacupuncture (EA). Eleven patients underwent MAC, and 13 patients received IGA. There were no instances of conversion from MAC to IGA, and both groups exhibited no major complications. The demographic characteristics, baseline parameters, and operative variables were comparable between the two groups.

RESULTS

Intraoperative opioid consumption was significantly lower in the MAC group compared to the IGA group (P < 0.001). The time to oral intake of liquids was significantly shorter in the MAC group (2.14 ± 0.90 h) compared to the IGA group (22.31 ± 3.33 h) (P < 0.001). Furthermore, the intensive care unit length of stay (ICU-LOS) and perioperative vasoactive-inotropic score (VIS) were significantly reduced in the MAC group compared to the IGA group (P < 0.001).

CONCLUSIONS

MAC emerges as a safe and viable alternative to general anesthesia for specific patient groups undergoing cardiac surgery with CPB. Furthermore, it may enhance postoperative recovery and minimize postoperative complications compared to IGA.

Safety and Effectiveness of Carbon Dioxide Removal CO2RESET Device in Critically Ill Patients

BACKGROUND

In this retrospective study, we report the effectiveness and safety of a dedicated extracorporeal carbon dioxide removal (ECCO₂R) device in critically ill patients.

METHODS

Adult patients on mechanical ventilation due to acute respiratory distress syndrome (ARDS) or decompensated chronic obstructive pulmonary disease (dCOPD), who were treated with a dedicated ECCO₂R device (CO2RESET, Eurosets, Medolla, Italy) in case of hypercapnic acidemia, were included. Repeated measurements of CO₂ removal (VCO₂) at baseline and 1, 12, and 24 h after the initiation of therapy were recorded.

RESULTS

Over a three-year period, 11 patients received ECCO₂R (median age 60 [43-72] years) 3 (2-39) days after ICU admission; nine patients had ARDS and two had dCOPD. Median baseline pH and PaCO₂ levels were 7.27 (7.12-7.33) and 65 (50-84) mmHg, respectively. With a median ECCO₂R blood flow of 800 (500-800) mL/min and maximum gas flow of 6 (2-14) L/min, the VCO₂ at 12 h after ECCO₂R initiation was 157 (58-183) mL/min. Tidal volume, respiratory rate, and driving pressure were significantly reduced over time. Few side effects were reported.

CONCLUSIONS

In this study, a dedicated ECCO₂R device provided a high VCO₂ with a favorable risk profile.

Non-intubated deep paralysis: a new anaesthesia strategy for vocal cord polypectomy

BACKGROUND

Vocal cord polyp is common otorhinolaryngological disease, traditionally treated by vocal cord polypectomy under a supporting laryngoscope with general anaesthesia. Although it is safe and controllable, it would cause some anaesthesia complications. Moreover, the complex process of general anaesthesia may significantly reduce surgical efficiency. Avoiding these problems remains an important issue.

METHODS

All patients were subjected to the standard non-intubated deep paralysis (NIDP) protocol consisting of four phases. An emergency plan was launched when NIDP cannot be implemented successfully. Patient characteristics, blood gas and monitoring data were collected during NIDP. Data concerning satisfaction, complications and duration of anaesthesia and recovery were collected to assess its effectiveness.

RESULT

Among 20 enrolled patients, the success rate of NIDP was 95%. Only one patient failed in completing NIDP. Blood gas analysis revealed that the partial pressure of oxygen and carbon dioxide was maintained at safe levels. Monitoring during NIDP revealed fluctuations in mean arterial pressure between 110 and 70 mmHg, and the heart rate was stable at 60-100 beats per minute. The duration of anaesthesia and postoperative recovery were 13.0 ± 2.84 and 5.47 ± 1.97 min, respectively. All patients and surgeons were satisfied with NIDP, and no complications were detected before discharge.

CONCLUSION

NIDP can be safely applied to patients and can replace general anaesthesia in vocal cord polypectomy. It can significantly reduce the duration of anaesthesia and postoperative recovery. No anaesthesia complications occurred without intubation, and patients and surgeons were satisfied with NIDP.

TRIAL REGISTRATIONS

This single-centre, prospective study was registered on clinicaltrial.gov (NCT04247412) on 30^th July 2020.

The Tissue Response to Hypoxia: How Therapeutic Carbon Dioxide Moves the Response toward Homeostasis and Away from Instability

Sustained tissue hypoxia is associated with many pathophysiological conditions, including chronic inflammation, chronic wounds, slow-healing fractures, microvascular complications of diabetes, and metastatic spread of tumors. This extended deficiency of oxygen (O₂) in the tissue sets creates a microenvironment that supports inflammation and initiates cell survival paradigms. Elevating tissue carbon dioxide levels (CO₂) pushes the tissue environment toward "thrive mode," bringing increased blood flow, added O₂, reduced inflammation, and enhanced angiogenesis. This review presents the science supporting the clinical benefits observed with the administration of therapeutic CO₂. It also presents the current knowledge regarding the cellular and molecular mechanisms responsible for the biological effects of CO₂ therapy. The most notable findings of the review include (a) CO₂ activates angiogenesis not mediated by hypoxia-inducible factor 1a, (b) CO₂ is strongly anti-inflammatory, (c) CO₂ inhibits tumor growth and metastasis, and (d) CO₂ can stimulate the same pathways as exercise and thereby, acts as a critical mediator in the biological response of skeletal muscle to tissue hypoxia.

Transcutaneous Carbon Dioxide Monitoring More Accurately Detects Hypercapnia than End-Tidal Carbon Dioxide Monitoring during Non-Intubated Video-Assisted Thoracic Surgery: A Retrospective Cohort Study

Transcutaneous carbon dioxide (PtcCO₂) monitoring is known to be effective at estimating the arterial partial pressure of carbon dioxide (PaCO₂) in patients with sedation-induced respiratory depression. We aimed to investigate the accuracy of PtcCO₂ monitoring to measure PaCO₂ and its sensitivity to detect hypercapnia (PaCO₂ > 60 mmHg) compared to nasal end-tidal carbon dioxide (PetCO₂) monitoring during non-intubated video-assisted thoracoscopic surgery (VATS). This retrospective study included patients undergoing non-intubated VATS from December 2019 to May 2021. Datasets of PetCO₂, PtcCO₂, and PaCO₂ measured simultaneously were extracted from patient records. Overall, 111 datasets of CO₂ monitoring during one-lung ventilation (OLV) were collected from 43 patients. PtcCO₂ had higher sensitivity and predictive power for hypercapnia during OLV than PetCO₂ (84.6% vs. 15.4%, p < 0.001; area under the receiver operating characteristic curve; 0.912 vs. 0.776, p = 0.002). Moreover, PtcCO₂ was more in agreement with PaCO₂ than PetCO₂, indicated by a lower bias (bias ± standard deviation; -1.6 ± 6.5 mmHg vs. 14.3 ± 8.4 mmHg, p < 0.001) and narrower limit of agreement (-14.3-11.2 mmHg vs. -2.2-30.7 mmHg). These results suggest that concurrent PtcCO₂ monitoring allows anesthesiologists to provide safer respiratory management for patients undergoing non-intubated VATS.

Lung-Protective Ventilation Attenuates Mechanical Injury While Hypercapnia Attenuates Biological Injury in a Rat Model of Ventilator-Associated Lung Injury

Background and Objective: Lung-protective mechanical ventilation is known to attenuate ventilator-associated lung injury (VALI), but often at the expense of hypoventilation and hypercapnia. It remains unclear whether the main mechanism by which VALI is attenuated is a product of limiting mechanical forces to the lung during ventilation, or a direct biological effect of hypercapnia.

Methods: Acute lung injury (ALI) was induced in 60 anesthetized rats by the instillation of 1.25 M HCl into the lungs via tracheostomy. Ten rats each were randomly assigned to one of six experimental groups and ventilated for 4 h with: 1) Conventional HighV_ENormocapnia (high V_T, high minute ventilation, normocapnia), 2) Conventional Normocapnia (high V_T, normocapnia), 3) Protective Normocapnia (V_T 8 ml/kg, high RR), 4) Conventional iCO₂Hypercapnia (high V_T, low RR, inhaled CO₂), 5) Protective iCO₂Hypercapnia (V_T 8 ml/kg, high RR, added CO₂), 6) Protective endogenous Hypercapnia (V_T 8 ml/kg, low RR). Blood gasses, broncho-alveolar lavage fluid (BALF), and tissue specimens were collected and analyzed for histologic and biologic lung injury assessment.

Results: Mild ALI was achieved in all groups characterized by a decreased mean PaO₂/FiO₂ ratio from 428 to 242 mmHg (p < 0.05), and an increased mean elastance from 2.46 to 4.32 cmH₂O/L (p < 0.0001). There were no differences in gas exchange among groups. Wet-to-dry ratios and formation of hyaline membranes were significantly lower in low V_T groups compared to conventional tidal volumes. Hypercapnia reduced diffuse alveolar damage and IL-6 levels in the BALF, which was also true when CO₂ was added to conventional V_T. In low V_T groups, hypercapnia did not induce any further protective effect except increasing pulmonary IL-10 in the BALF. No differences in lung injury were observed when hypercapnia was induced by adding CO₂ or decreasing minute ventilation, although permissive hypercapnia decreased the pH significantly and decreased liver histologic injury.

Conclusion: Our findings suggest that low tidal volume ventilation likely attenuates VALI by limiting mechanical damage to the lung, while hypercapnia attenuates VALI by limiting pro-inflammatory and biochemical mechanisms of injury. When combined, both lung-protective ventilation and hypercapnia have the potential to exert an synergistic effect for the prevention of VALI.

Review of pediatric hypercarbia and intraoperative management

PURPOSE OF REVIEW

Hypercarbia in pediatric patients is an important component of intraoperative management. Despite marked advances in medicine and technology, it is uncertain what the physiological CO2 range in neonates, infants and small children. This data is extrapolated from the adult population. We are going to review advantages and disadvantages of CO2 measurement techniques, causes and systemic effects of hypercarbia. We are going to discuss how to approach management of intraoperative hypercarbia.

RECENT FINDINGS

Although physiological range in this patient population may not be fully understood, it is known that any rapid change from a child's baseline increases risks of complications. Any derangements in CO2 are further compromised by hypoxia, hypotension, hypothermia, anemia, all of which may occur in a dynamic operating room environment.

SUMMARY

Pediatric anesthesiologists and their teams must remain vigilant and anticipate these developments. Care must be taken to avoid any rapid changes in these vulnerable patients to minimize risks of adverse outcomes.

Hypercapnia from Physiology to Practice

Acute hypercapnic ventilatory failure is becoming more frequent in critically ill patients. Hypercapnia is the elevation in the partial pressure of carbon dioxide (PaCO₂) above 45 mmHg in the bloodstream. The pathophysiological mechanisms of hypercapnia include the decrease in minute volume, an increase in dead space, or an increase in carbon dioxide (CO₂) production per sec. They generate a compromise at the cardiovascular, cerebral, metabolic, and respiratory levels with a high burden of morbidity and mortality. It is essential to know the triggers to provide therapy directed at the primary cause and avoid possible complications.

The 6-Minute Walk Test and Anthropometric Characteristics as Assessment Tools in Patients with Obstructive Sleep Apnea Syndrome. A Preliminary Report during the Pandemic

Patients with obstructive sleep apnea syndrome (OSAS) exhibit low cardio-fitness impact, attributed to fragmented sleep architecture and associated pathophysiological sequelae. The purpose of our study was to investigate fitness indicators during 6-min walk test (6MWT) and oxidative stress markers in apnea-hypopnea index (AHI) in OSAS patients stratified by severity. A total of 37 newly diagnosed patients, comorbidity-free, were divided into two groups: (Moderate OSAS (n = 12), defined as ≥ 15 AHI < 30 events per hour; Age: 50.7 ± 7.2 years, BMI: 32.5 ± 4.0 kg/m2 vs. Severe OSAS (n = 25), defined as AHΙ ≥ 30 events per hour; Age: 46.3 ± 10.4 years, BMI: 33.3 ± 7.9 kg/m2). Measurements included demographics, anthropometric characteristics, body composition, blood sampling for reactive oxygen metabolites' levels (d-ROM) and plasma antioxidant capacity (PAT), and followed by a 6MWT. AHI was significantly associated with d-ROMs levels, chest circumference in maximal inhalation and exhalation (Δchest), neck circumference, as well as 6MWT-derived indices. In conclusion, our study determines bidirectional interrelationships between OSAS severity and anthropometrics, body composition, and fitness metrics. These findings indicate that the impact of OSAS should be evaluated well beyond polysomnography-derived parameters.

Extracorporeal Carbon Dioxide Removal in the Treatment of Status Asthmaticus

OBJECTIVES

Venovenous extracorporeal carbon dioxide removal may be lifesaving in the setting of status asthmaticus.

DESIGN

Retrospective review.

SETTING

Medical ICU.

PATIENTS

Twenty-six adult patients with status asthmaticus treated with venovenous extracorporeal carbon dioxide removal.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Demographic data and characteristics of current and prior asthma treatments were obtained from the electronic medical record. Mechanical ventilator settings, arterial blood gases, vital signs, and use of vasopressors were collected from the closest time prior to cannulation and 24 hours after initiation of extracorporeal carbon dioxide removal. Extracorporeal carbon dioxide removal settings, including blood flow and sweep gas flow, were collected at 24 hours after initiation of extracorporeal carbon dioxide removal. Outcome measures included rates of survival to hospital discharge, ICU and hospital lengths of stay, duration of invasive mechanical ventilation and extracorporeal carbon dioxide removal support, and complications during extracorporeal carbon dioxide removal. Following the initiation of extracorporeal carbon dioxide removal, blood gas values were significantly improved at 24 hours, as were peak airway pressures, intrinsic positive end-expiratory pressure, and use of vasopressors. Survival to hospital discharge was 100%. Twenty patients (76.9%) were successfully extubated while receiving extracorporeal carbon dioxide removal support; none required reintubation. The most common complication was cannula-associated deep venous thrombosis (six patients, 23.1%). Four patients (15.4%) experienced bleeding that required a transfusion of packed RBCs.

CONCLUSIONS

In the largest series to date, use of venovenous extracorporeal carbon dioxide removal in patients with status asthmaticus can provide a lifesaving means of support until the resolution of the exacerbation, with an acceptably low rate of complications. Early extubation in select patients receiving extracorporeal carbon dioxide removal is safe and feasible and avoids the deleterious effects of positive-pressure mechanical ventilation in this patient population.

Influence of Intermittent Hypoxia/Hypercapnia on Atherosclerosis, Gut Microbiome, and Metabolome

Obstructive sleep apnea (OSA), a common sleep disorder characterized by intermittent hypoxia and hypercapnia (IHC), increases atherosclerosis risk. However, the contribution of intermittent hypoxia (IH) or intermittent hypercapnia (IC) in promoting atherosclerosis remains unclear. Since gut microbiota and metabolites have been implicated in atherosclerosis, we examined whether IH or IC alters the microbiome and metabolome to induce a pro-atherosclerotic state. Apolipoprotein E deficient mice (ApoE-/- ), treated with IH or IC on a high-fat diet (HFD) for 10 weeks, were compared to Air controls. Atherosclerotic lesions were examined, gut microbiome was profiled using 16S rRNA gene amplicon sequencing and metabolome was assessed by untargeted mass spectrometry. In the aorta, IC-induced atherosclerosis was significantly greater than IH and Air controls (aorta, IC 11.1 ± 0.7% vs. IH 7.6 ± 0.4%, p < 0.05 vs. Air 8.1 ± 0.8%, p < 0.05). In the pulmonary artery (PA), however, IH, IC, and Air were significantly different from each other in atherosclerotic formation with the largest lesion observed under IH (PA, IH 40.9 ± 2.0% vs. IC 20.1 ± 2.6% vs. Air 12.2 ± 1.5%, p < 0.05). The most differentially abundant microbial families (p < 0.001) were Peptostreptococcaceae, Ruminococcaceae, and Erysipelotrichaceae. The most differentially abundant metabolites (p < 0.001) were tauro-β-muricholic acid, ursodeoxycholic acid, and lysophosphoethanolamine (18:0). We conclude that IH and IC (a) modulate atherosclerosis progression differently in distinct vascular beds with IC, unlike IH, facilitating atherosclerosis in both aorta and PA and (b) promote an atherosclerotic luminal gut environment that is more evident in IH than IC. We speculate that the resulting changes in the gut metabolome and microbiome interact differently with distinct vascular beds.

Acidic preconditioning reduces lipopolysaccharide-induced acute lung injury by upregulating the expression of angiotensin-converting enzyme 2

Acid preconditioning (APC) through carbon dioxide inhalation can exert protective effects during acute lung injury (ALI) triggered by ischemia-reperfusion. Angiotensin-converting enzyme 2 (ACE2) has been identified as a receptor for severe acute respiratory syndrome coronavirus and the novel coronavirus disease-19. Downregulation of ACE2 plays an important role in the pathogenesis of severe lung failure after viral or bacterial infections. The aim of the present study was to examine the anti-inflammatory mechanism through which APC alleviates lipopolysaccharide (LPS)-induced ALI in vivo and in vitro. The present results demonstrated that LPS significantly downregulated the expression of ACE2, while APC significantly reduced LPS-induced ALI and provided beneficial effects. In addition, bioinformatics analysis indicated that microRNA (miR)-200c-3p directly targeted the 3'untranslated region of ACE2 and regulated the expression of ACE2 protein. LPS exposure inhibited the expression of ACE2 protein in A549 cells by upregulating the levels of miR-200c-3p. However, APC inhibited the upregulation of miR-200c-3p induced by LPS, as well as the downregulation of ACE2 protein, through the NF-κB pathway. In conclusion, although LPS can inhibit the expression of ACE2 by upregulating the levels of miR-200c-3p through the NF-κB pathway, APC inhibited this effect, thus reducing inflammation during LPS-induced ALI.

Hypercapnia Regulates Gene Expression and Tissue Function

Carbon dioxide (CO₂) is produced in eukaryotic cells primarily during aerobic respiration, resulting in higher CO₂ levels in mammalian tissues than those in the atmosphere. CO₂ like other gaseous molecules such as oxygen and nitric oxide, is sensed by cells and contributes to cellular and organismal physiology. In humans, elevation of CO₂ levels in tissues and the bloodstream (hypercapnia) occurs during impaired alveolar gas exchange in patients with severe acute and chronic lung diseases. Advances in understanding of the biology of high CO₂ effects reveal that the changes in CO₂ levels are sensed in cells resulting in specific tissue responses. There is accumulating evidence on the transcriptional response to elevated CO₂ levels that alters gene expression and activates signaling pathways with consequences for cellular and tissue functions. The nature of hypercapnia-responsive transcriptional regulation is an emerging area of research, as the responses to hypercapnia in different cell types, tissues, and species are not fully understood. Here, we review the current understanding of hypercapnia effects on gene transcription and consequent cellular and tissue functions.

Hypercapnia: An Aggravating Factor in Asthma

Asthma is a common chronic respiratory disorder with relatively good outcomes in the majority of patients with appropriate maintenance therapy. However, in a small minority, patients can experience severe asthma with respiratory failure and hypercapnia, necessitating intensive care unit admission. Hypercapnia occurs due to alveolar hypoventilation and insufficient removal of carbon dioxide (CO₂) from the blood. Although mild hypercapnia is generally well tolerated in patients with asthma, there is accumulating evidence that elevated levels of CO₂ can act as a gaso-signaling molecule, triggering deleterious effects in various organs such as the lung, skeletal muscles and the innate immune system. Here, we review recent advances on pathophysiological response to hypercapnia and discuss potential detrimental effects of hypercapnia in patients with asthma.

Correlation of Arterial CO2 and Respiratory Impedance Values among Subjects with COPD

Chronic obstructive pulmonary disease (COPD) is a respiratory illness characterized by airflow limitation and chronic respiratory symptoms with a global prevalence estimated to be more than 10% in 2010 and still on the rise. Furthermore, hypercapnic subject COPD leads to an increased risk of mortality, morbidity, and poor QoL (quality of life) than normocapnic subjects. Series of studies showed the usefulness of the forced oscillation technique (FOT) to measure small airway closure. Traditional findings suggested that hypercapnia may not be the main treating targets, but recent findings suggested that blood stream CO₂ may lead to a worse outcome. This study aimed to seek the relationship between CO₂ and small airway closure by using FOT. Subjects with COPD (n = 124; hypercapnia 22 and normocapnia 102) were analyzed for all pulmonary function values, FOT values, and arterial blood gas analysis. Student’s t-test, Spearman rank correlation, and multi linear regression analysis were used to analyze the data. COPD subjects with hypercapnia showed a significant increase in R5, R20, Fres, and ALX values, and a greater decrease in X5 value than normocapnic patients. Also, multiple linear regression analysis showed R5 was associated with hypercapnia. Hypercapnia may account for airway closure among subjects with COPD and this result suggests treating hypercapnia may lead to better outcomes for such a subject group.

Hypercapnia Exacerbates the Blood-Brain Barrier Disruption Via Promoting HIF-1a Nuclear Translocation in the Astrocytes of the Hippocampus: Implication in Further Cognitive Impairment in Hypoxemic Adult Rats

Hypercapnia in combination with hypoxemia is usually present in severe respiratory disease in the intensive care unit (ICU) and can lead to more severe cognitive dysfunction. Increasing evidence has indicated that the compromised blood–brain barrier (BBB) in the hippocampus in hypoxemia conditions can result in cognitive dysfunction. However, the role and underlying mechanism of hypercapnia in the BBB disruption remains poorly known. A rat model of hypercapnia was first established in this study by intubation and mechanical ventilation with a small-animal ventilator. After this, the cognitive function of the experimental rats was assessed by the Morris water maze test. The BBB permeability was evaluated by the Evans Blue (EB) test and brain water content (BWC). Western blot analysis was carried out to detect the protein expressions of total and nuclear hypoxia-inducible factor-1α (HIF-1α), matrixmetalloproteinase-9 (MMP-9) and Aquaporins-4 (AQP-4) in the hippocampus tissue. Double immunofluorescence further verified the protein expression of different biomarkers was localized in the astrocytes of the hippocampus. Hypercapnia alone did not disrupt the BBB, but it could further enhance the BBB permeability in hypoxemia. Concomitantly, up-regulation of nuclear HIF-1α, AQP-4, MMP-9 protein expression along with increased degradation of the occludin and claudin-5 proteins was found in the hypercapnia rat model, while the total HIF-1α remained unchanged. Interestingly, these changes were independent of the acidosis induced by hypercapnia. Of note, after premedication of 2-Methoxyestradiol (2ME2, an inhibitor of HIF-1α nuclear translocation), the disrupted BBB could be restored resulting in improvement of the cognitive impairment. Meanwhile, accumulation of nuclear HIF-1α, protein expression of AQP-4 and MMP-9 and protein degradation of the occludin and claudin-5 were decreased. Thus, our study demonstrated that hypercapnia can further disrupt the BBB through promoting HIF-1α nuclear translocation and up-regulation of AQP-4 and MMP-9 in hypoxemia. It is therefore suggested that the cascade of hypercapnia-induced nuclear HIF-1α protein translocation in hypoxia-activated astrocytes may be a potential target for ameliorating cognitive impairment.

Respiratory Physiology for the Anesthesiologist

Respiratory function is fundamental in the practice of anesthesia. Knowledge of basic physiologic principles of respiration assists in the proper implementation of daily actions of induction and maintenance of general anesthesia, delivery of mechanical ventilation, discontinuation of mechanical and pharmacologic support, and return to the preoperative state. The current work provides a review of classic physiology and emphasizes features important to the anesthesiologist. The material is divided in two main sections, gas exchange and respiratory mechanics; each section presents the physiology as the basis of abnormal states. We review the path of oxygen from air to the artery and of carbon dioxide the opposite way, and we have the causes of hypoxemia and of hypercarbia based on these very footpaths. We present the actions of pressure, flow, and volume as the normal determinants of ventilation, and we review the resulting abnormalities in terms of changes of resistance and compliance.

Ventilatory frequency during intraoperative mechanical ventilation and postoperative pulmonary complications: a hospital registry study

BACKGROUND

High ventilatory frequencies increase static lung strain and possibly lung stress by shortening expiratory time, increasing intrathoracic pressure, and causing dynamic hyperinflation. We hypothesised that high intraoperative ventilatory frequencies were associated with postoperative respiratory complications.

METHODS

In this retrospective hospital registry study, we analysed data from adult non-cardiothoracic surgical cases performed under general anaesthesia with mechanical ventilation at a single centre between 2005 and 2017. We assessed the association between intraoperative ventilatory frequency (categorised into four groups) and postoperative respiratory complications, defined as composite of invasive mechanical ventilation within 7 days after surgery or peripheral oxygen desaturation after extubation, using multivariable logistic regression. In a subgroup, we adjusted analyses for arterial blood gas parameters.

RESULTS

A total of 102 632 cases were analysed. Intraoperative ventilatory frequencies ranged from a median (inter-quartile range [IQR]) of 8 (8-9) breaths min^-1 (Group 1) to 15 (14-18) breaths min^-1 (Group 4). High ventilatory frequencies were associated with higher odds of postoperative respiratory complications (adjusted odds ratio=1.26; 95% confidence interval, 1.14-1.38; P<0.001), which was confirmed in a subgroup after adjusting for arterial partial pressure of carbon dioxide and the ratio of arterial oxygen partial pressure to fractional inspired oxygen. We identified considerable variability in the use of high ventilatory frequencies attributable to individual provider preference (ranging from 22% to 88%) and temporal change; however, the association with postoperative respiratory complications remained unaffected.

CONCLUSIONS

High intraoperative ventilatory frequency was associated with increased risk of postoperative respiratory complications, and increased postoperative healthcare utilisation.

Hypercapnia Impairs Na,K-ATPase Function by Inducing Endoplasmic Reticulum Retention of the β-Subunit of the Enzyme in Alveolar Epithelial Cells

Alveolar edema, impaired alveolar fluid clearance, and elevated CO2 levels (hypercapnia) are hallmarks of the acute respiratory distress syndrome (ARDS). This study investigated how hypercapnia affects maturation of the Na,K-ATPase (NKA), a key membrane transporter, and a cell adhesion molecule involved in the resolution of alveolar edema in the endoplasmic reticulum (ER). Exposure of human alveolar epithelial cells to elevated CO2 concentrations caused a significant retention of NKA-β in the ER and, thus, decreased levels of the transporter in the Golgi apparatus. These effects were associated with a marked reduction of the plasma membrane (PM) abundance of the NKA-α/β complex as well as a decreased total and ouabain-sensitive ATPase activity. Furthermore, our study revealed that the ER-retained NKA-β subunits were only partially assembled with NKA α-subunits, which suggests that hypercapnia modifies the ER folding environment. Moreover, we observed that elevated CO2 levels decreased intracellular ATP production and increased ER protein and, particularly, NKA-β oxidation. Treatment with α-ketoglutaric acid (α-KG), which is a metabolite that has been shown to increase ATP levels and rescue mitochondrial function in hypercapnia-exposed cells, attenuated the deleterious effects of elevated CO2 concentrations and restored NKA PM abundance and function. Taken together, our findings provide new insights into the regulation of NKA in alveolar epithelial cells by elevated CO2 levels, which may lead to the development of new therapeutic approaches for patients with ARDS and hypercapnia.

The quality of acute intensive care and the incidence of critical events have an impact on health-related quality of life in survivors of the acute respiratory distress syndrome - a nationwide prospective multicenter observational study

Background: Initial treatment (ventilator settings, rescue therapy, supportive measures), and prevention of critical events improve survival in ARDS patients, but little data exists on its effect on health-related quality of life (HRQOL) and return to work (RtW) in survivors. We analyzed the association of the intensity of treatment at ARDS onset and the incidence of critical events on HRQOL and RtW a year after ICU discharge.

Methods: In a prospective multi-centre cohort study, the intensity of treatment and the incidence of critical events were determined at 61 ICUs in Germany. At 3, 6, and 12 months, 396 survivors reported their HRQOL (Short-Form 12) and RtW. The parameters of the intensity of acute management (lung protective ventilation, prone position, hemodynamic stabilization, neuromuscular blocking agents), and critical events (hypoxemia, hypoglycemia, hypotension) were associated with HRQOL and RtW.

Results: Patients ventilated at ARDS onset with a low tidal volume (VT≤7 ml/kg) had higher arterial carbon dioxide levels (PaCO₂=57.5±17 mmHg) compared to patients ventilated with VT>7ml/kg (45.7±12, p=0.001). In a multivariate adjusted dichotomized analysis, a better mental 3-month SF-12 was observed in the higher VT-group (mean 43.1±12) compared to the lower VT-group (39.5±9, p=0.042), while a dichotomized analysis for driving pressures (≤14 mbar vs >14 mbar) did not show any difference neither in PaCO₂ levels nor in HRQOL parameters. A decrease in the mental (6-month: 40.0±11 vs 44.8±13, p=0.038) and physical SF-12 (12-month: 38.3±11 vs 43.0±13, p=0.015) was reported from patients with hypoglycemia (blood glucose <70 mg/dl) compared to those without hypoglycemic episodes. More frequent vasopressor use with mean arterial pressure ≥65 mmHg was associated with an impaired physical SF-12 (6-month: 38.8±10) compared to less vasopressor use (43.0±11, p=0.019).

Conclusions: In acute management of ARDS, a lower VT strategy associated with hypercapnia, as well as the frequent usage of catecholamines and the management of blood glucose may influence short-term HRQOL of survivors. The awareness of these findings is of clinical importance for the acute and post-ICU care.

An observational study of end-tidal carbon dioxide trends in general anesthesia

PURPOSE

Despite growing evidence supporting the potential benefits of higher end-tidal carbon dioxide (ETCO₂) levels in surgical patients, there is still insufficient data to formulate guidelines for ideal intraoperative ETCO₂ targets. As it is unclear which intraoperative ETCO₂ levels are currently used and whether these levels have changed over time, we investigated the practice pattern using the Multicenter Perioperative Outcomes Group database.

METHODS

This retrospective, observational, multicentre study included 317,445 adult patients who received general anesthesia for non-cardiothoracic procedures between January 2008 and September 2016. The primary outcome was a time-weighted average area-under-the-curve (TWA-AUC) for four ETCO₂ thresholds (< 28, < 35, < 45, and > 45 mmHg). Additionally, a median ETCO₂ was studied. A Kruskal-Wallis test was used to analyse differences between years. Random-effect multivariable logistic regression models were constructed to study variability.

RESULTS

Both TWA-AUC and median ETCO₂ showed a minimal increase in ETCO₂ over time, with a median [interquartile range] ETCO₂ of 33 [31.0-35.0] mmHg in 2008 and 35 [33.0-38.0] mmHg in 2016 (P <0.001). A large inter-hospital and inter-provider variability in ETCO₂ were observed after adjustment for patient characteristics, ventilation parameters, and intraoperative blood pressure (intraclass correlation coefficient 0.36; 95% confidence interval, 0.18 to 0.58).

CONCLUSIONS

Between 2008 and 2016, intraoperative ETCO₂ values did not change in a clinically important manner. Interestingly, we found a large inter-hospital and inter-provider variability in ETCO₂ throughout the study period, possibly indicating a broad range of tolerance for ETCO₂, or a lack of evidence to support a specific targeted range. Clinical outcomes were not assessed in this study and they should be the focus of future research.

Anesthesia for nonintubated video-assisted thoracic surgery

PURPOSE OF REVIEW

This review focuses primarily on nonintubated video-assisted thoracic surgery (NIVATS), and discusses advantages, indications, anesthetic techniques, and approaches to intraoperative crisis management.

RECENT FINDINGS

Advancements in endoscopic, endovascular, and robotic techniques have expanded the range of surgical procedures that can be performed in a minimally invasive fashion. For thoracic operations in particular, video-assisted thoracic surgery (VATS) has largely replaced traditional thoracotomy, and continued technical development has made surgical access into the pleural space even less disruptive. As a consequence, the need for general anesthesia and endotracheal intubation has been re-examined, such that regional or epidural analgesia may be sufficient for cases where lung collapse can be accomplished with spontaneous ventilation and an open hemithorax. This concept of NIVATS has gained popularity, and in some centers has now expanded to include procedures involving placement of multiple ports. Although still relatively uncommon at present, a small number of randomized trials and meta-analyses have indicated some advantages, suggesting that NIVATS may be a desirable alternative to general anesthesia with endotracheal intubation for specific indications.

SUMMARY

Although anesthesia for NIVATS is associated with some of the same risks as general anesthesia with endotracheal intubation, NIVATS can be successfully performed in carefully selected patients.

Nonintubated-Awake Anesthesia for Uniportal Video-Assisted Thoracic Surgery Procedures

Nonintubated video-assisted thoracic surgery (VATS) strategies are gaining popularity. This review focuses on noninutbated VATS, and discusses advantages, indications, anesthetic techniques, and approaches to intraoperative crisis management. Advances in endoscopic, endovascular, and robotic techniques have expanded the range of surgical procedures that can be performed in a minimally invasive fashion. The nonintubated thoracoscopic approach has been adapted for use with major lung resections. The need for general anesthesia and endotracheal intubation has been reexamined, such that regional or epidural analgesia may be sufficient for cases where lung collapse can be accomplished with spontaneous ventilation and an open hemithorax.

Importance of carbon dioxide in the critical patient: Implications at the cellular and clinical levels

Important recent insights have emerged regarding the cellular and molecular role of carbon dioxide (CO₂) and the effects of hypercapnia. The latter may have beneficial effects in patients with acute lung injury, affording reductions in pulmonary inflammation, lessened oxidative alveolar damage, and the regulation of innate immunity and host defenses by inhibiting the expression of inflammatory cytokines. However, other studies suggest that CO₂ can have deleterious effects upon the lung, reducing alveolar wound repair in lung injury, decreasing the rate of reabsorption of alveolar fluid, and inhibiting alveolar cell proliferation. Clearly, hypercapnia has both beneficial and harmful consequences, and it is important to determine the net effect under specific conditions. The purpose of this review is to describe the immunological and physiological effects of carbon dioxide, considering their potential consequences in patients with acute respiratory failure.

Quantitative analysis of the effect of end-tidal carbon dioxide on regional cerebral oxygen saturation in patients undergoing carotid endarterectomy under general anaesthesia

AIMS

Regional cerebral oxygen saturation (rSO₂ ) is currently the most used measure in clinical practice to monitor cerebral ischaemia in patients undergoing carotid endarterectomy (CEA). Although end-tidal carbon dioxide (P_ET CO₂ ) is known as a factor that influences rSO₂ , the relationship between P_ET CO₂ and rSO₂ has not been quantitatively evaluated in patients with severe arteriosclerosis. This study aimed to evaluate the effect of P_ET CO₂ on rSO₂ in patients undergoing CEA under general anaesthesia.

METHODS

The intervention to change P_ET CO₂ was conducted between skin incision and clamping of the carotid artery. The rSO₂ values were observed by changing P_ET CO₂ in the range of 25-45 mmHg. The P_ET CO₂ -rSO₂ relationship was characterized by population analysis using a turnover model.

RESULTS

In total, 1651 rSO₂ data points from 30 patients were used to determine the pharmacodynamic characteristics. Hypertension (HTN) and systolic blood pressure (SBP) were significant covariates on the slope factor in the stimulatory effect of P_ET CO₂ on rSO₂ and fractional turnover rate constant (k_out ), respectively. The estimates of the parameters were k_out (min^-1 ): 3.59 for SBP <90 mmHg and 0.491 for SBP ≥90 mmHg, slope: 0.00321 for patients with HTN and 0.00664 for patients without HTN.

CONCLUSION

The presence of HTNattenuates the response of rSO₂ after a change in P_ET CO₂ . When cerebral blood flow is in a state of decline caused by a decrease in SBP to <90 mmHg, the response of rSO₂ to P_ET CO₂ is increased. It is advisable to maintain SBP >90 mmHg in patients with HTNduring CEA.

Hypercapnic acidosis attenuates pressure-dependent increase in whole-lung filtration coefficient (Kf)

Hypercapnic acidosis (HCA) has beneficial effects in experimental models of lung injury by attenuating inflammation and decreasing pulmonary edema. However, HCA increases pulmonary vascular pressure that will increase fluid filtration and worsen edema development. To reconcile these disparate effects, we tested the hypothesis that HCA inhibits endothelial mechanotransduction and protects against pressure-dependent increases in the whole lung filtration coefficient (K_f). Isolated perfused rat lung preparation was used to measure whole lung filtration coefficient (K_f) at two levels of left atrial pressure (P_LA = 7.5 versus 15 cm H₂O) and at low tidal volume (LV_t) versus standard tidal volume (STV_t) ventilation. The ratio of K_f2/K_f1 was used as the index of whole lung permeability. Double occlusion pressure, pulmonary artery pressure, pulmonary capillary pressures, and zonal characteristics (ZC) were measured to assess effects of HCA on hemodynamics and their relationship to K_f2/K_f1. An increase in P_LA2 from 7.5 to 15 cm H₂O resulted in a 4.9-fold increase in K_f2/K_f1 during LV_t and a 4.8-fold increase during STV_t. During LV_t, HCA reduced K_f2/K_f1 by 2.7-fold and reduced STV_t K_f2/K_f1 by 5.2-fold. Analysis of pulmonary hemodynamics revealed no significant differences in filtration forces in response to HCA. HCA interferes with lung vascular mechanotransduction and prevents pressure-dependent increases in whole lung filtration coefficient. These results contribute to a further understanding of the lung protective effects of HCA.

Effects of hypercapnia on the lung

Gases are sensed by lung cells and can activate specific intracellular signalling pathways, and thus have physiological and pathophysiological effects. Carbon dioxide (CO₂ ), a primary product of oxidative metabolism, can be sensed by eukaryotic cells eliciting specific responses via recently identified signalling pathways. However, the physiological and pathophysiological effects of high CO₂ (hypercapnia) on the lungs and specific lung cells, which are the primary site of CO₂ elimination, are incompletely understood. In this review, we provide a physiological and mechanistic perspective on the effects of hypercapnia on the lungs and discuss the recent understanding of CO₂ modulation of the alveolar epithelial function (lung oedema clearance), epithelial cell repair, innate immunity and airway function.

[The old man and the I sea U : Essay on faith, fate and evidence - after the manner of Hemingway]

Robert Bartlett, emeritus Professor of surgery at the University of Michigan in Ann Arbor, USA, transformed classical works of world literature (Charles Dickens: A Christmas Carol, Lewis Carroll: Alice in Wonderland) into teaching aids for advanced training in intensive care medicine. He recently turned his hand to the well-known work of Ernest Hemingway: the Nobel Prize winning novel The Old Man and the Sea. Subsequent to Robert Bartlett's essay this article provides background information and comments on the current problems in modern intensive care medicine addressed in his essay.

Respiratory Acid-Base Disorders in the Critical Care Unit

The incidence of respiratory acid-base abnormalities in the critical care unit (CCU) is unknown, although respiratory alkalosis is suspected to be common in this population. Abnormal carbon dioxide tension can have many physiologic effects, and changes in Pco₂ may have a significant impact on outcome. Monitoring Pco₂ in CCU patients is an important aspect of critical patient assessment, and identification of respiratory acid-base abnormalities can be valuable as a diagnostic tool. Treatment of respiratory acid-base disorders is largely focused on resolution of the primary disease, although mechanical ventilation may be indicated in cases with severe respiratory acidosis.

A 47-year-old woman with hypercapnia and altered mental status

Permissive hypercapnia is a commonly used ventilator strategy in attempt to improve refractory hypoxemia. The rationale of such practice is based on the assumption that hypercapnia, although associated with altered mental status, is well tolerated. Here, we report a case in which the altered mental status caused by hypercapnia is underlined by a life-threatening mechanism. The case indicates the severity of hypercapnia may have been mistakenly overlooked in the past.

Hypercapnia Suppresses the HIF-dependent Adaptive Response to Hypoxia

Molecular oxygen and carbon dioxide are the primary gaseous substrate and product of oxidative metabolism, respectively. Hypoxia (low oxygen) and hypercapnia (high carbon dioxide) are co-incidental features of the tissue microenvironment in a range of pathophysiologic states, including acute and chronic respiratory diseases. The hypoxia-inducible factor (HIF) is the master regulator of the transcriptional response to hypoxia; however, little is known about the impact of hypercapnia on gene transcription. Because of the relationship between hypoxia and hypercapnia, we investigated the effect of hypercapnia on the HIF pathway. Hypercapnia suppressed HIF-α protein stability and HIF target gene expression both in mice and cultured cells in a manner that was at least in part independent of the canonical O2-dependent HIF degradation pathway. The suppressive effects of hypercapnia on HIF-α protein stability could be mimicked by reducing intracellular pH at a constant level of partial pressure of CO2 Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase that blocks lysosomal degradation, prevented the hypercapnic suppression of HIF-α protein. Based on these results, we hypothesize that hypercapnia counter-regulates activation of the HIF pathway by reducing intracellular pH and promoting lysosomal degradation of HIF-α subunits. Therefore, hypercapnia may play a key role in the pathophysiology of diseases where HIF is implicated.